This disclosure is related to the field of geophysical measuring instruments. More specifically, the disclosure relates to geophysical measuring instruments requiring good acoustic coupling to the ground, such as seismic sensors.
Seismic sensors require good acoustic coupling to the ground, which may be characterized by a sensing element that detects ground motion with insignificant loss of signal or change in the characteristics of the vibration as it is transferred from the ground to the sensing element. When a sensing element (e.g., a geophone or other particle motion responsive sensor) is disposed in loose surface soils, for example, the detected signal quality may be diminished relative to that which would be detected if the sensing element were disposed in compact soil due to particle distribution of energy resulting from increased degrees of freedom. Additionally, the rapid expansion of computing power affords the opportunity to record seismic activity with greater precision. Therefore, the ability to record changes in seismic signal content (“seismic signatures”) over time intervals of months or years has become feasible. However, the requirement to service seismic sensors from time to time has made it difficult to reinstall a seismic sensor into the exact location of the original recording and/or in the identical orientation. The latter limitation may make time lapse recording of directionally sensitive seismic energy, e.g., shear energy difficult.
Referring to FIGS. 1 and 2, International Patent Application Publication No. WO 92/19991 describes an autonomous seismic sensing and data recording system 1 disposed in an auger shaped housing 2. The shape of the housing 2 is intended to improve acoustic coupling between the ground and the housing 2 by having external threads 21 in FIG. 2 such that the ground is spread apart as the housing 2 is rotated and moved axially into the ground by the action of the threads 21. A tool receptacle 5 (FIG. 1) disposed on an upper cover 3 of the housing 2 maybe provided to enable threading and unthreading of the housing 2 for installation and removal of the sensor system 1. The foregoing system 1 is illustrated in cross section in FIG. 1, showing internal sensing (orthogonally mounted geophones 7, 8 and 9 and magnetometers 10A, 10B) and signal processing/recording devices 11, 12, 14, 14, 15, 16, 17. FIG. 2 shows the housing 2 with auger type threads 21 in oblique view. The threads have a selected pitch P, thread height H and maximum external diameter Ds. In this example the housing 2 and sensor components (see FIG. 1) are an inseparable unit. Therefore, removal of the sensor from the ground requires retrieval of the housing 2, thus making it necessary to attempt to locate the original sensor position in certain types of surveys requiring precise subsequent location of sensors in their former positions. While the ability to mark a sensor location with markers such as flags is possible, such markers are prone to displacement or loss.
One aspect of accurate time lapse or 4D imaging separated by months or years has been the practice of removing the sensor device for subsequent use in different locations until it is time to record data with the sensors in their original positions. In some instances, one or more components of the sensor system may require servicing, e.g., recharging batteries, or the sensor system may be required to be removed from the ground in order to interrogate an internal data storage device. It is thus necessary to remove the sensor system housing from the ground. In some cases such removal may be undesirable because over time, acoustic coupling between such a sensor housing and the ground may actually improve. Also to provide a consistent image, it is essential that the sensor occupy the exact same location on the earth for every imaging process. It is desirable, therefore, to have a sensor mount that provides equivalent acoustic coupling to the ground as that disclosed in the WO '991 publication while enabling removal and replacement of the active components of the sensor system and leaving an acoustic coupling mechanism in place at the selected ground location.